This invention relates to fuel control systems for gas turbine aero engines and is particularly concerned with fuel control systems for use in aircraft capable of landing and taking-off vertically.
The British Aerospace P.L.C. Harrier aircraft is powered by a Rolls-Royce P/C Pegasus engine which employs vectorable hot and cold propulsive thrust nozzles. When landing vertically the nozzles are pointed downwards and the fuel flow is adjusted to control the upward thrust. By adjusting the upward thrust, vertical speed of the aircraft can be changed and the aircraft can be landed in a controlled manner. At the instant that the undercarriage wheels contact a firm base, some of the weight of the aircraft is transferred to the wheels and consequently the upward thrust momentarily exceeds the weight of the aircraft. Unless this thrust is diminished rapidly the aircraft is prone to "jump" upwards causing problems, for example, on wet decks. During descent, pilots are trained to detect "touch-down" and slam the throttles shut immediately. If the pilot or the fuel control system responds too slowly, the aircraft may bounce.
One current form of fuel control system employs a digital electronic control unit (DECU) which monitors various engine parameters (such as pressures, temperatures, speeds of rotation of spools etc.) and is responsive to movements of the pilot's throttle lever, to operate a motor driven valve which controls the flow of fuel to the engine. It is customary to provide an independently operable manual control system which enables the pilot to override the DECU controlled system. In general, the manual flow control valve (MFC) is inoperative when the DECU controlled system is operating and a change over valve is provided. Nevertheless the MFC is linked to the movement of the pilot's throttle lever and for most of the time the MFC is being moved in synchronism with the pilot's lever but is redundant because it is not controlling fuel flow. Due to the high work load and the intense concentration required during landing, it is not practical for the pilot to switch to manual control during landing.